Desiliconization of ferrophosphorus



Patented July 19,1949

DESILICONIZATION OF FERROPHOSPHORUS Bethune G. Klugh, Anniston, Ala., assignor to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware v No Drawing. Application April 29, 1947,

V Serial No. 744,797

9 Claims.

The present invention relates to ferrophosphorus of substantially reduced silicon content and to an improved method of manufacturing same. More particularly, the invention is concerned with the production of substantially silicon-free ferrophosphoruswhich is particularly suitable for use in the steel industry.

Ferrophosphorus is used in the production of steel products requiring rephosphorization, such as rimmed steel. This specific character of steel is essential in a number of important finished products, particularly thin steel sheets which require a small phosphorus content for effecting the required stiffness. Rimmed steel diifers in character and method of production from that general type called killed steel, the primary difierence being that the rimmed steel must have the ingots cast without a deoxidizing or killing agent of the ferro-silicon class. This practice in regard to rimmed steel has been definitely established by substantially all of the experts in the art and consequently they demand a ferrophosphorus with as low silicon content as possible.

There are certain types of steels which can use a high silicon ferrophosphorus for their purpose, but low silicon ferrophosphorus can be used equally as well and hence the steel industry prefers a product of sufficiently low silicon content for rimmed steel so that a single stock of this alloy will be adapted to all its uses.

Commercially, ferrophosphorus is produced as a by-product of the thermal method of manufacturing elemental phosphorus or phosphoric acid which involves heating phosphate rock, a silicious flux and a carbonaceous reducing agent in an electric or blast furnace. By properly adjusting the furnace and burden conditions, it is possible to directlyproduce ferrophosphorus having a low content of silicon, but this method of operation is inconsistent with low maintenance, high phosphorus recovery and readily controlled furnace operation. Consequently, it is a more economical practice to produce ferrophosphorus having a relatively high silicon content and then effect a separation of the silicon by a subsequent treatment.

It has already been proposed to mix molten ferrophosphorus rich in silicon and obtained inthe volatilization of phosphorus with fresh quantities of raw phosphate, preferably tricalcium phosphate, and then fuse the resulting mixture in a non-oxidizing atmosphere, whereby the phosphate is reduced by the silicon contained in the ferrophosphorus. The silica formed as a result of this reaction combines with the line liberated from the phosphate to produce a silicate slag and the phosphorus liberated by the reduction is partially taken up bythe ferrophosphorus, thereby increasing the phosphorus concentration in the alloy. The elemental phosphorus which does not combine with the alloy escapes and is condensed or burnt to form phosphoric acid in the conventional manner.

The above method has achieved some measure of success, but it has been by no means entirely satisfactory, principally because of the difficulty of bringing about the following reaction without effecting substantial dephosphorization "or the ferrophosphorus: 1

For example, a competing reaction which may occur in the high temperature contact of phosphate material with ferrophosphorus is that of the accepted form in the elimination of phosphorus in steel manufacture. This reaction occurs in two steps as follows:

2P+8FeO=3FeO.PtO5l-5Fe (3) FeO.P2Os+3CaO=3Ca0.P205-[-3Fe0 Net reaction of (2) and (3):

- It is to be noted that in the above phosphorus removal reactions the reactant material is not tricalcium phosphate, but FeO and CaO. However, both of these oxides occur in efiective proportions in the phosphate materials available and hence reaction 4 must be avoided if satisfactory silicon removal is to be obtained without substantial dephosphorization of the ferrophosphorus.

Moreover, it can be demonstrated by thermal calculations that reaction 4 is exothermic to the degree of 3334 P. C. U. per pound of phosphorus phosphate flows from the fact that ferrophosphorus has a higher density than the phosphate material and consequently it separates from the latter before the reaction goes to completion. This difficulty arises'not only when both materials, ferro-phosphorus and phosphate material, are mixed together and heated up to the reaction temperature from the cold state but also when the reaction is carried out by contacting molten ferrophosphorus with cold'phosphate material. As a result of this difliculty, ferrophosphorus having a variable silicon content. is. produced which is objectionable for obvious reasons.

The primary object of the present invention is, therefore, to provide a process of desilic'onizing ferrophosphorus in which the above difficulties and disadvantages are either completely eii' nated or substantially reduced.

Another object of; theinvention is to provide an I economical and -eff ective; method of, remo ving silicon from ferrophosphorus without impairing the original desirable;characteristics of the alloy. 1

Anaddition al object-is to .provide acommer cially feasible method-of; desiliconizing ferrophosphorus while ,maintaining "or increasing its phosphorus; content;

A further object ;is .;to provideuaprocess ,of desiliconizing ferrophosphorusin which the desired reaction between the silicon offerrophosphorus and tricalcium phosphate takes place to the substantial exclusion of the competing reaction hereinbefore described.

A still further object is to provide alow silicon ferrophosphorus which is eminently suitable for use in the'steelindustry;

Other objects and advantages will be apparent to those skilled inftheiart as 'the description proceeds? I r In accordance calcium phosphate, phosphate rock or equivalent phosphate material lspreli'atedto a temperature of at least 2500 F. and then contacted in a suitable furnace with silicon-containing ferrophosphorus, preferablyinthe molten condition, to form a silicate-slag and .ferrophosphorus of increased phosphorus content and V substantially reduced silicon content, whereupon these products are successively tapped from the furnace in the conventional manner.

As indicated inth'e preceding paragraph, it is preferred to employferrophosphorus in the molten conditionas it :canbe handlediand treated in this condition :with greater. facility. However, it is within the scopev of .thzpresent invention to employ the alloy in the;cold;state. In charging the cold alloy into the furnace, it is essential to successful operation that the phosphate reacting material be preheated to a temperature sufiicient to combine the CaO, FeO and S102 content thereof into-silicates; that there beample fusion of the phosphate material and that the fused phosphate material be ofsufficient depthjo immerse the charged alloy., In .practice, it is preferred to charge the cold alloy inthe finely divided'state andjin small increments inorder that its submergence in the fused reactantlbe assured.

The preheating step is a critical feature of the invention andshould be carried out at a temperature of at least 2500,F.5as a temperature of this magnitudefis required to complete'the reaction of, SiO2 and the free CaO and Fe'zO'zof the phosphate material into silicates. "This'step achieves two important functions which result in the production of. a'desiliconized product having an inwith the present invention, tri- 2':

creased phosphorus content instead of a similar product of decreased phosphorus content. First of all, by bringing about the formation of the above stable silicates, it prevents reaction 4 from taking place when the phosphateg'materialjis; contacted with the ferrophospl'io'rus.- In addition, it ensures complete or substantially complete reaction between the silicon content of the ferrophosphorus and the phosphate. material as the latter has no cold spots to which the ferrophosphorus can penetrate and be cooledit'o a point below the desiliconization temperature.

The desiliconization of ferrophosphorus is effected by reacting its silicon content with tricalcium phosphate in at least the theoretical proportions deflned in reaction 1, but in order to ensure maximum silicon removal, it is preferred to employ an excess ofijphosphate material. In f c in i w, o :theq ev rei or ion:p obl m involved, S l eh ese nze ce s. ofrnos: ete i e wh ch... sl rs nosen t e'i n tinn as a liner forthe-furna ce o the e i th ts ve In carrying; on t reaction, the a e:fingiifimphfimml fi' Q JId;'be brought er.underleqnsl iensi nsurinaumformity of heating and 1 the 'maxi-mum possible atc b contact 7 between s verberatory best accomplished in th furnace, but it :is to M R the invention lstnot re ric t The invention is ill e o atann -1 ierren ibsrhp by weight 9 s l sen and 5. phorus was heated topa. tern rature; ofi from 2710 to 6 "v r nesi h edfintp; alq einanzow rcall i m mma-11,

81 lbs. of phosphate material-was introduced into a circular, revrberatory furnace 3in' 'such a manner as to'provideithereinian inverted 'coni'cal container: space. V 257.1bs50f finely '-'divi'ded ferro phosphorus waslthen' chargedzlritotthe furnace and arranged in: the;.containercspacerintth'e form'- of a conical pile oftheiailoyir' Afterzchargingfthe furnace in the above:mannersthireactants were heated by means of a gas flame in accordance with thefollowingschedple:

The results obtained by the above heating operation are indicated below:

Per Cent Per Cent 1? Bi Ferr hos horns Char ed To Reverbetatory Fl g 22.95 2.74 Ferrophosphorus Product of Treatment 19. 94 0.

Example III 150 lbs. of ferrophosphorus was melted in an electric furnace and tapped at .a temperature of from 2900 F. to 2980 F. into a narrow elongated reverberatory furnace containing phosphate concentrates which were preheated to 2500 F. and arranged in the form of a trough for the molten metal. Upon completing the tapping operation, the molten metal in contact with the concentrates was heated to a temperature of from 2590 F. to 2770" F. for 1 hour and 58 minutes by means of a natural gas-hot air blast to yield a calcium silicate slag and ferrophosphorus of increased phosphorus content and relatively low silicon content. The extent of the desiliconization and phosphorization of the ferrophosphorus is indicated by the following data:

Per cent Per cent P Si Ferrcphosphorns Charged To Reverberetory Furnace 20. 55 4. 26 Fcrrophosphorus Product of Treatment 23. 52 0. 10

Per cent Per cent P Si Ferrophosphorus Charged To Reverberatory Furnace 21. 17 3. 80 Ferrophosphorus Product of Treatment 21. 21 3. 22

The method of desiliconizing ferrophosphorus hereinbefore described yields a product of greatly increased commercial importance over the original material. Thus, the desiliconized product has a higher content of phosphorus than the original material and hence less of it is required to supply the same quantity of phosphorus. Moreover, since it contains less than 1% by weight of silicon, it is particularly suitable for use by the steel industry as now a single stock of this alloy will be adapted to all its uses, including the production of rimmed steel.

The various operating conditions of the present process of desiliconizing ferrophosphorus will now be summarized.

In the desiliconization reaction,it is essential 6 temperature of at least 2500 F., and preferably above that temperature, to produce stable silicates from the SiOz and the free CaO and FaO contained in the phosphate material. This has the effect of preventing reaction 4 from taking place when the phosphate material is contacted with molten ferrophosphorus; it also facilitates substantially complete desiliconization by elimihating cold spots within the phosphate material which reduce the temperature of the higher grav ity ferrophosphorus to a point below the desiliconization reaction temperature. 7 y

In addition, it is essential that there be suflicient molten phosphate material to cover the ferrophosphorus introduced for contact and reaction therewith. The ferrophosphorus should also have'adequate time and contact with the phosphate material in order to complete the re-' action for removal of all or substantially all of the silicon in the metal.

The ferrophosphorus is preferably in the molten condition when brought into contact with the phosphate material, but as indicated earlier herein it may be employed in the cold state provided the alloy is added to the fused phosphate material in the finely divided state and insuch amounts and under such conditions as to be immersed in the fused material or at least be supplied'with a film thereof. These requirements must be met in order to achieve successful operation as Harry unfused alloy is exposed to combustion or oxidizing gases, the formation of FeO takes place which accelerates reaction 4 and thereby results in dephosphorization of the alloy.

In operating in accordance with the preferred procedure, it is desirable that the ferrophosphorus be of high fusion temperature, that is,'at least as high as 2540" F. and under no circumstances, whether before or after supplemental heating, should the temperature of the ferrophosphorus or the phosphate material be low enough to reduce the resultant temperature be-. low 2400" F. and preferably not below 2500 F.

Thus, if the phosphate material is initially preheated to a temperature of at least 2500 F., the desiliconization reaction may be carried out at a temperature of from 2400 F. up to 3300" F. or highertemperature, starting with either cold or molten ferrophosphorus.

As hereinbefore indicated, the phosphate material may beused in substantially the theoretical proportions required by Equation 1, but it is desirable to use an excess in order to ensure substantially complete removal of silicon. In fact. in view of the corrosion problems involved, it is preferable to use an excess of phosphate which is large enough to provide a liner for the furnace or reactor. The thickness of the liner will vary widely with the operation conditions, but in general, a liner thickness of about 12 inches has been found to be satisfactory. However, where sulficiently resistant equipment is available, no advantage from the standpoint of desiliconization is achieved by. using greater than 8% excess of phosphate material.

The time requiredv for completion of treatment of a given quantity of ferrophosphorus is largely governed by the reaction temperature and the relative area of contact of the molten metal and the molten phosphate material entering the reaction. Under the conditions prevailing in the present process where the previously heated phosphate material in the furnace is largely in fluid form, the molten alloy entering the furnace will that the phosphate material be preheated to a have interfacial reactant contact completely enasses e veloping the metal section of the bath-and-hence theereaction will'proceed atahigh rate. In fact, under the preferred operating; conditions, where the reaction is carried out in areverberatoryfurnace' with maximum surface contact and'at relatively' high temperature that is; above-260W F., the desiliconization reaction will takeplace-withina pe-rio'd of several minutes.

With reference to the apparatus, it is des-ir ablethatthe phosphate mater-ialsiuncti'on' a'sy the bottom and sidewalls ofthe furnace ore reactor in the desiliconization reactiomand thewharging;

means; for: the phosphate '=materi'als::mi1st"bei'of such-arrangement that the portion consumedin the smelting operation willbe automaticallysre placed; This requirement may he met hy-thelmse' of Ta la'rge excess of phosphate #ma'terial the initial furnacetcharge oriby'providingrfongravity filling from storage intothe furnace:

The furnace hearth formed "by the 'phosphate' reactant material Within the furnacefproper'm'ust provide "fora relatively narrow and 'shallow "re ceptacle" space withinwhich the molten fer-rephosphoruswill flow, so that the maximum ob tainable area of contact of the metalandractant' phosphate in solid; pastyl. or liquid' state is available; for the required reaction;

The entire area in. which: the reactionproceeds must be uniformly heated to a temperature with" higher temperature so as "to avoidinactive or dormant pockets. This is important for the' followingzreasons:

If there is any small-seetioniln which the' tem' perature is bel'cw that "required forthe de's'i-liconization reaction; the 'alloyaiwill not be-dc'siii' conized; to the desired degree.

Ifia 'portionof the reactant phospnate matenai does" not reach the temperature requi-r'eo to coin plet'ethe reaction f' SiO'2 'ancl the'freeCaQ -afid FeG content thereof into 'st'able silicatea tlie'y will is quite fluid atthe temperatures prevailing in the present 'desilicdniz'altion processand it'f'wlll ten-3i to flom' thus producing the desii'edagitatidn' of the reacting mass'i" It" is'desjirable, however, to supplement this actionby"blovlfrig the "601115115": tron gas'e'sin a tangential direction to the surface in order to effect 'th"'g1"afie'st 'p6Si-51 movement;

The above requirements are provided ini afreverbe'ra'tory type of furnacej particularl one of circular arrangement. greatest "attainable uniformity 'of'jhea't' transfer conditions fronigas' flames. mus-un rinph'at'e material is charged peripherallyarounil the side wall and in'the centerfof the-furnace,

This "fornprowaesitne thereby "producing 'a trough receptacle -foij the melting' 'aridliduidphase reactions. 'rhephos' irate 'fill'ii err'e'ets'the replac ment ofcons'umfw reactant, "and"in'i addition rmctionsjftnrouenout asthef'contaifir wall 1' an' '"d bottomirofi tliqiciuid- 81: mater-lat; during-a the course of the; conversion. Although a reverberatory type-of furnaceis preferred for use in the present desiliconization proeess, any other suitable furnace, including an eleetric furnace, may be employed which meets the'aboverequirements-as-to-uniformity of heat conditions andefficient surface contact between the reactants.

The expression phosphate material? as used' in. the specification and claims is intended to enter til r 'Iciurri phosphate, raw phosphate rock, plaos p'h e, fockl concentrate; phosphate sinter andl rnf-ixturesfthereofor equivalent materials.

WhileI have 'cl'escribe'di above certain" specific foiiifsof nyinventiafi'; it will'be appreciated that it is susceptible to various changesahdmodiflcations without departingirom the spirit thereof.

Iclairn: v

1. The process vvhichf comprises contacting ee-ssh material preheated to" a"ternperature of at le'a's't "2509 F. with ferrophosphorus "rich in silic nan'd therebyforming a silicate 'slag'a'nd ferrophosphoru's of, relatively low silicon content.

2; The" process which comprises contacting phoph'at material preheated"toat'least'2500 E; with. molten ferrophosphorusfrichin silicon and therety ronnihg asilicate slag and ferrophosphorus of; relatively meanest content, said preheated phosphate material and said" fer'ro phosphorus having a temperature of su'fiicient magnitude to maintain" the resultant eaction temperature at -a value of at "least 2400" F.

3. The process which I coniprises contacting phosphate material preheated to'at least 2500 F., with molten ferrcphosphorus rich in silicon to form'a silicate slag and ferrophosphorus of increased phosphoruscontent andsubstantially decreased siliconconten't, saidphosphate material being eni'ployedin substantially the theoretical proportions "required to oxidize the silicon con tent of said ferrophosphorus rich in silicon.

ili'I'he" process which comprises contacting phosphateimaterial preheated to at least 2500 ngwnnmonen ferrophosphdrus rich in silicon to rorn'iia'snicate slag and ferrophospfioriis of. illcreasedjpnosphorus content and'subsi'lantially'"decfasdi'silicori content, said phosphate material being: employed in an amount 'not eicceding ai iout Y8 eicces'sbf 'the'theor'etical proportions quired to oxidize the silicon content ofsaid 'ferr'o phosphorus rich in silicon.

5. The process which comprises contacting in a revcrberatory type furnace phosphate material preheated --toa temperature of at least 2500 F'.,

with molten ferrophosphorus rich'in siliconto form aj silicate slag and ferrophosphorus' of increased phospherus' edntentana substantially de creased 's iiicda'coct eatgaad thereupon successive- 1ytapping said slag'and said alloypr'oduct' "from sammrnace':

5., The focess' wh ehcomprises tappingfm olten ferrophosjphorius" rich "in' silicon onto phosphate agnet preheated" to 'a temperature of at least 2506" F. and tnerjymaintaming "the reactants at a ."te'rhp'erat'ure 'of at"ieas't' 2400 F. until substantially c mpete uesuieomzauon has taken'plac'e.

7;Th p cess'tvmehcomprisestatpmg inolten p us rich in' silicon" onto phosphate material preheated "to 'a"teniperatur' of at least 2500 F.. and. theri 'rnaint'a ii'ng the reactants at a 'teinp'eiaturehflatleast' 24 F. by means of a gas fla'r'iieuiitil' sulistaritially complete. desiliconi zati on-has takenplace. I I V 8.1 Tfiiioe's'sfihioh comprises tapping molten ferrophosphorus rich in silicon into a reverberatory type furnace lined with phosphate material which has been preheated to a temperature of at least 2500 F. and then heating the reactants to a temperature above 2600 F. to yield a silicate slag and ferrophosphorus of increased phosphorus content and substantially decreased silicon content.

9. The process which comprises tapping molten ferrophosphorus rich in silicon into a reverberatory type furnace lined with at least partially fused phosphate material and then heating the reactant materials to a temperature of at least 2400 F. to form a silicate slag and ferrophosphorus of increased phosphorus content and substantially decreased silicon content, said partially fused phosphate material being employed in 10 an amount suificient to provide said tapped ferrophosphorus with at least a thin coating of said phosphate material.

BEI'HUNE G. KLUGH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Great Britain Aug. 27, 1931 

